An epoxy resin-based photosensitive resin composition has heretofore been used as an optical waveguide core layer-forming material intended for a mixed flexible printed circuit board for optical/electrical transmission, and in formation of the pattern of a core layer using the composition, a desired core pattern has been produced by performing photoirradiation through a photomask. In the patterning through the curing of such core layer-forming material, a curing reaction through irradiation with an i-line (wavelength: 365 nm) has been generally performed, and for example, a photoacid generator having a triphenylsulfonium cation having a wide π-conjugated area of a cation moiety has been used for the purpose of improving the sensitivity of the material.
However, the use of such photoacid generator as described above shows the following tendency. A by-product (dead body) to be incorporated after the photo fragmentation of the photoacid generator itself also has a wide conjugated area and absorbs light in a short-wavelength region, and hence an optical waveguide loss increases. The foregoing tendency has been a technical problem involved in an optical waveguide material using a photocurable resin (Patent Literature 1).